Firstly you need to make sure you already have International Roaming activated simply by login to your selfcare website, click on Subscription and look out for the button International Roaming. If this part is not turn on, turn this on. Deposit of RM300 may apply.
Once you are in the country, just turn on roaming in your phone, either on your ios or android os.
You can refer to the link and read all up about roaming here.